1. Field of the Invention
The present invention generally relates to a layout of a circuit board, in particular, to a layout of a circuit board capable of changing impedance values at both ends of one transmission line.
2. Description of Related Art
Circuit boards have been widely applied in various electronic devices. Many different electronic components are mounted on a circuit board, and are electrically connected to each other through the layout of the circuit board. Moreover, the heat generated by the electronic components is partially dissipated by metal portions of the circuit boards. In actual application, the mainboard of a notebook computer is an application of the circuit board with various electronic components mounted thereon.
In order to adjust an equivalent impedance at both ends of a segment of a transmission line in the circuit board with respect to a ground layer or power layer located beside the transmission line to a particular value. Conventionally, the line thickness, line width, or line pitch of the segment of the transmission line, or the distance between the segment of the transmission line and the adjacent reference plane thereof may be changed to achieve the purpose of adjusting the equivalent impedance at both ends of the segment of the transmission line.
However, in order to increase the equivalent impedance of the segment of the transmission line with respect to a ground layer or power layer, the line thickness, line width, or line pitch of the segment of the transmission line in the circuit board cannot be reduced under the limitations of the process accuracy and the process yield. Moreover, restricted by the thickness of the circuit board, the distance between the segment of the transmission line and the adjacent reference plane thereof cannot be increased. Therefore, the current layout of a circuit board is limited in design.